In a semiconductor process, a titanium oxide film is widely used as a dielectric film or an insulating film, and there is a process for forming a titanium oxide film on silicon. This process is used, for example, in a technique called double patterning to form a fine pattern below a resolution limit of photolithography. In double patterning, for example, a core material made of Si is formed with a predetermined pattern width using photolithography, and a titanium oxide film serving as an uppermost hard mask is formed thereon. Subsequently, the titanium oxide film is etched to form sidewall spacers made of a titanium oxide film on the sidewall of the core material made of silicon. Next, the core material is removed by etching so that sidewall spacers made of a titanium oxide film remain as a hard mask. By using this as an etching mask, a film to be processed, which is formed using the same procedure, is anisotropically etched. This makes it possible to etch the film to be processed with a pattern width which is half the pattern width of the core material.
There are methods for forming titanium oxide using atomic layer deposition (ALD), in which a TiCl4 gas and an oxidizing agent are alternately supplied to form a film. For example in a related art, a titanium oxide film is formed by a plasma ALD process in which a TiCl4 gas, which is a Ti raw material gas, and plasma of an oxygen-containing gas, such as an O2 gas, are alternately supplied.
However, in the method described above, although a titanium oxide film is obtained at a high film formation rate, an interfacial oxide film (a SiO2 film) is formed to be relatively thick in a silicon portion after the formation of the titanium oxide film. Therefore, when the titanium oxide film is used as a hard mask for etching a predetermined film, there is a possibility that an error will occur in the pattern dimension of the film to be etched.